1. Field of the Invention
The present invention relates to an electrophotographic photosensitive element suitable to production of a printing plate for use in lithographic offset printing (referred to as offset printing hereinunder) and a process for producing the printing plate (referred to as a master hereinunder).
2. Description of Related Art
In the conventional simple printing techniques suitable to produce a smaller amount of prints with a number of plates, printing masters are generally manufactured by the so-called direct processing of block plates through an electrophotographic process and used to perform offset printing as is well known in the art. Since such techniques can produce relatively easily and rapidly printed materials, they have been widely employed. Recently, in view of both quality and quantity, the simple printing techniques have found greatly increasing use in a wide variety of printings such as commercial printings of bills, catalogs and brochures as well as conventional plain printings of leaflets and various documents in the firms. With such an increase of demand and a tendency to diversity, there is a commercial need to further improve the quality of printings and to increase the speed of printing. A variety of methods for manufacturing the masters by using the electrophotographic process have been known. One of the most widely used methods comprises providing zinc oxide powder as a photoconductor, dispersing said powder into an insulating binder resin to produce a coating dispersion, applying the dispersion onto an electrically conductive substrate and drying the same to produce a photosensitive element. Generally, the element is successively subjected to electrically charging, exposure imagewise to the light through the original to produce latent image and development of the latent image with toner to produce a plate having a toner pattern on the photosensitive layer. The plate having the toner pattern (referred to as a developed plate hereinunder) is processed with a desensitizing agent to render the non-image portions lipophobic (referred to as a lipophobicating agent hereinunder), whereby a master is produced.
Zinc oxide widely used as photoconductor as described above is generally sensitized primarily with xanthene dyes. In this case, the photosensitive elements are colored with the sensitizing dyes to be the so-called pink master sheets which are widely employed. The pink master sheets, however, have a poor smoothness on the level of the top surfaces (in the image area ), a poor granularity, a lower tendency to produce sharp image printings, and a lower resistance to printing abrasion with a mass production of printings being impractical. Moreover, the pink master sheets are difficult to inspect, that is, to effect the so-called plate-inspection due to their coloration and calls for particular care in modification and editing of the original. As above, the current electrophotographic processes for producing printing plates and techniques for manufacturing the masters in the field of printings are in need of improvement.